1. Field of the Invention
The present invention relates to a gas sensor, and to its method of manufacture. It relates particularly, but not exclusively, to an electrochemical gas sensor for sensing carbon monoxide (CO) gas.
2. Prior Art
An electrochemical gas sensor for sensing an oxidisible or reducible gas, such as carbon monoxide, usually includes a sensing electrode, a counter electrode and a diffusion barrier. The diffusion barrier allows gas to be sensed, to pass to the sensing electrode. In one type of gas sensor, as described, for example, in the Applicant's copending International Patent Application No. WO-A1-9614576, the sensing and counter electrodes are located on a gas permeable membrane and are in contact with an electrolyte.
In terms of physical construction, electrochemical gas sensors usually comprise an external housing, which acts as a reservoir for electrolyte; a wick, to keep the electrolyte in contact with the electrodes; and external electrical terminals, which make electrical contact with the electrodes.
During operation of the aforementioned gas sensor, an electrochemical reaction occurs at the sensing electrode with the gas to be sensed, and a reaction also occurs with oxygen at the counter electrode. Electric current is carried through the electrolyte by ions produced in these reactions, and the amount of current indicates the concentration of the gas being sensed. A further electrode (the reference electrode) may be employed, for example, in combination with a potentiostat circuit, to maintain a constant potential difference between the sensing electrode and the electrolyte. This increases the stability of operation of the gas sensor. Electrodes are connected to external current sensors via electrical terminals.
External electrical terminals are usually formed from brass or copper pins. Brass and copper both react with the acid electrolyte, and so the gas sensor has to be specially designed so that the pins do not come into contact with the electrolyte. Platinum does not react with acid, and so platinum strips can be used to form an electrical path between the electrodes and external sensors and/or an external electricity supply. However, platinum strips are commonly placed in a seal region between the housing and the gas permeable membrane, and electrolyte can leak from this region. Platinum is also expensive, and so gas sensors having platinum terminals are expensive to manufacture.
Another example of a gas sensor is described in U.S. Pat. No. 5,314,605 (Dragerwerk). The aforementioned US Patent describes a gas permeable region through which holes have been formed. Electrodes pass through the holes. No matter how carefully the region between the periphery of each hole and the electrode is sealed, there is a risk of electrolyte leaking through this seal.